Generally, in vapor phase sterilization, a liquid sterilant is metered from a reservoir or other container into a vaporizer or sterilization chamber in which vaporization occurs. To ensure effective and efficient sterilization, the liquid should be metered in accurately and reproducibly measured amounts.
Several different methods have been proposed for metering liquid sterilant into a vaporization system. In one approach, a cassette having a group of sealed cells is coupled to a sterilization chamber by dispensing apparatus. Each cell contains a predetermined dose of liquid sterilant. After the sterilization chamber is evacuated, the cells are punctured sequentially, and their contents forced into the evacuated chamber by pneumatic pressure.
Because the amount of sterilant injected is limited to the cell volume, or multiples thereof, the foregoing approach is not flexible. It also is not practical or economical for use in multi-phase or flow-through sterilization cycles, which would require multiple cassettes. Further, shelf-life problems arise, for example, when the system is employed to dispense small amounts (e.g., a few mls) of hydrogen peroxide sterilant. During storage, the hydrogen peroxide is prone to degrade into gases or vapors, which may rupture the cassette cells, unless vented. Venting, however, reduces the sterilant concentration over time.
In other known proposals, a dispensing pump propels the liquid sterilant directly from a reservoir into a vaporizer, through dispensing lines. Liquid metering is accomplished by various known methods, including: a) controlling the volume dispensed per pump stroke; b) controlling the revolution rate of a continuous flow, fixed output pump; and c) controlling the dispensing time period from a continuous flow, fixed output pump. Alternatively, metering is achieved by mounting the liquid reservoir on an electronic balance, and then monitoring the weight loss as the liquid is pumped from the reservoir.
In a further approach, liquid sterilant is metered into the vaporizer by controlling the time period dispensing occurs at a fixed, controlled pressure or vacuum level. Again, the liquid sterilant is carried directly from the reservoir into the vaporizer, through dispensing lines.
The previously proposed pump/pressure dispensing methods may perform satisfactorily, within their given dispensing capabilities and accuracies, when the liquid does not degrade into or otherwise generate vapors or gases during storage or handling. However, when gases or vapors are produced from liquid retained in the dispensing equipment, the performances of such methods can be adversely affected.
For example, in the prior methods which meter by controlling operating parameters of a fixed rate, volumetric pump, entrained air bubbles (and other gases/vapors) prevent the sterilant liquid from being accurately and reproducibly metered, because the pump cannot distinguish between the liquid sterilant and air bubbles. Further, if equipment incorporating a stroke-type pump is allowed to sit idle for an extended period of time, air bubbles forming in the lines, valves, and filters may even prevent the pump from operating, i.e., the system will "vapor lock."
Similarly, air bubbles adversely affect the performance of methods which meter liquid sterilant by controlling the dispensing time period at a fixed pressure or vacuum, because the liquid is pushed or sucked into the vaporizer, along with the air bubbles, in a non-uniform matter.
Dispensing accuracy may also be reduced in systems which monitor weight loss from the liquid reservoir, when such systems sit idle for several hours. Weight loss from the reservoir, as measured by the balance, does not account for the air bubbles formed in the dispensing lines, which are dispensed into the vaporizer at start-up. To enhance dispensing accuracy, the dispensing lines can be purged prior to injection, to replace any remaining liquid having entrained air bubbles with substantially pure liquid. This has been accomplished by directing a high rate of liquid flow through the dispensing lines and back into the reservoir, with a diverter valve. This procedure does not entirely avoid measuring problems created by air bubbles, however, where the pump sucks entrained air bubbles back into the liquid reservoir and into the dispensing lines, during the purge step.
Metering problems caused by air bubbles are aggravated when the liquid sterilant is injected into the vaporizer in intermittent pulses, because the smaller increments injected require better resolution. Also, bubbles build-up between pulses and steady state conditions are not achieved.
Other problems are presented by the prior metering methods, which dispense liquid sterilant directly from the reservoir to the vaporizer. In general, when employing a sterilant such as hydrogen peroxide, which breaks down over time, high injection rates and pressures (or vacuums) are desired, to ensure that the sterilant is moved quickly through the vaporizer to the intended sterilization site. However, high dispensing pressures may also give rise to increased system leaks. The dispensing equipment must be constructed from materials which can physically withstand such high pressures and yet retain material compatibility with the liquid sterilant.
Further, the measuring resolution of the system is reduced at higher dispense rates and pressures. This problem is compounded when the increased liquid agitation which accompanies high delivery speeds and pressures generates additional air bubbles.
In the prior metering methods, if the pressure (or suction) or dispense rate is reduced to provide a lower liquid flow, in an attempt to increase metering resolution and reduce system leaks, the time period for injecting amounts of liquid into the vaporizer (and through to the sterilization chamber) is undesirably increased. Further, when liquid sterilant is metered by monitoring the dispensing time period at a fixed pressure or vacuum, it has been determined that the dispense rate fluctuates with the liquid sterilant level in the reservoir.
There is a need for a method of metering liquid sterilant from a reservoir into a vaporization system, in accurately and reproducibly measured amounts, particularly where the liquid vapor forms gases or vapors during storage and handling. There is also a need for a metering method which can deliver the measured liquid sterilant into the vaporizer at higher pressures and speeds, while avoiding system leaks and material compatibility problems.